Urban Heat, Urban Legends

According to news reports, the recent heat wave in California resulted in about 150 deaths. The United Nations’ Intergovernmental Panel on Climate Change predicts that global warming will exacerbate the problem dramatically, doubling or tripling the number of heat-related fatalities in North American cities in the next decade. The UN is dead wrong, because it assumes what climate researchers call the “Stupid People Hypothesis”: that people will simply sit around and fry to death without doing anything to beat the heat.

Global warming or not, our cities are warming, and will continue to do so. Sprawling masonry and blacktop retain heat, and the density of urban construction prevents wind from cooling it off. (Here in D.C., there’s an additional warming effect: waste heat from all the money changing hands.)

But heat and heat-related deaths are not synonymous. In fact, in several refereed papers published in recent years, my Virginia colleague Robert Davis and I demonstrated that heat-related deaths have, in aggregate, declined significantly as our cities have warmed. In fact, in a statistical sense, we have completely engineered heat-related mortality out of several of our urban cores, particularly in eastern cities like Philadelphia.

Considering every decade of mortality data at once is misleading; examining it decade-by-decade is more informative. When looked at sequentially, the data reveals a remarkable adaptation: as cities have warmed, the “threshold” temperatures at which mortality begins to increase have also risen — more than the temperatures of the cities.

For example, in Philadelphia in the 1960s, mortality began to increase once the high temperature exceeded 83 degrees. In the 1970s, the mortality threshold rose to the low 90s. In the last decade, there has been little evidence for any threshold at which mortality increases. In other words, people have adapted to their changing climate.

How? Instead of simmering, people buy air conditioning. Every level of government warns of the danger of excessive exposure to heat, and people seek out cooler places.

Social adaptation can take place very quickly. In mid-July 1995, over 500 people died from an intense weekend heat wave in Chicago. Research by University of Illinois climatologist Michael Palecki, published in the Bulletin of the American Meteorological Society in 2001, shows that a 1999 Chicago heat wave of comparable intensity resulted in only 15 percent as many deaths.

This summer’s heat is a bit unusual. Usually, when it’s very hot in the eastern U.S., temperatures are normal or below average in the West, or vice-versa. This year it’s hot everywhere.

Is history repeating itself, or is global warming at work? It’s hard to say. Several summers in the 1930s were known for intense heat across the nation. Nineteen-thirty was a scorcher: in rural Virginia, far from Washington’s sprawl, people suffered a total of 21 triple-digit days. Even with the excess heat contributed by the growth of the city, Washington currently averages only one 100 degree day per year.

The fact is that we cannot completely discriminate between repetitive history and prospective warming when it comes to a single summer. The better place to look for warming is in the winter. Greenhouse-effect theory predicts that the coldest temperatures of winter will rise much more sharply than the hottest ones of summer. And indeed, for the last several decades, winter’s lows have warmed out of proportion to summer’s highs.

All of which illustrates the complexity of global warming. Would people accept — even welcome? — climate change that greatly alleviated winter discomfort at the cost of slightly higher summer temperatures?

Clearly, people have adapted to the heat. The evidence shows that, the warmer the city, the more quickly its residents adapt. Heat-related deaths are increasing in only one major American city: chilly Seattle. San Francisco and Los Angeles, two other cities that are relatively cool in the summer compared to those to their east, show no change in mortality.

As the UN’s climatologists should recognize, heat waves are dangerous when they are rare and unexpected, because people are unfamiliar with them and slow to take appropriate actions to minimize their exposure. As heat waves become more common, we will simply be better prepared for them and incorporate them into our daily lives and routines — just as the people of Phoenix and Dallas and Houston and New Orleans do, every summer day. Because they’re not stupid.